The invention relates to a method of forming a multi-color image and more specifically to an electro(photo)graphic or magnetographic color imaging system in which color separation images are formed utilizing colored toner powders, in at least the secondary colors of magenta (red/blue), cyan (green/blue) and yellow (red/green) and the color separation images are combined to produce a multi-color image.
In electrostatography and electrophotography, just as in other printing techniques such as planographic and intaglio printing, multi-color images are obtained by forming color separation images by means of transparent colored material in the colors of magenta, cyan and yellow and transferring these color separation images, possibly together with a separation image in black, to an image receiving material in registration.
The separation images are formed by generating charge patterns successively in accordance with each separation image on a dielectric or photoconductive element, and developing the respective charge patterns with the associated colored transparent toner powder of magenta, cyan and yellow.
To develop the successive charge patterns, three different two-component developer powders are used, each comprising carrier particles and transparent colored toner particles in the colors of magenta, cyan and yellow, respectively. The composition of these developer powders is so selected that the toner particles are charged tribo-electrically against the carrier particles to a charge polarity which is opposite to that of the charge pattern to be developed. A method of this kind together with the apparatus for performing the method are disclosed inter alia in U.S. Pat. No. 2,986,466 and UK patent 1,074,147.
The developer powders used in this method have to satisfy a large number of requirements. For example, they must all have substantially the same development characteristic, which means that their tribo-electric behavior must be substantially the same under all conditions which may occur in the multi-color printer. The dyes in the toner powders must dissolve completely in the resin used in the toner powder, or be very finely distributed therein to produce transparent toner powder. They must also have sufficiently high stability and suitable absorption characteristics for use in multi-color reproduction. Since the various dyes often influence the tribo-electric properties of the toner powder in different ways, it is difficult to make up different toner powders for combined use in a multi-color reproduction system of the type referred to above. A frequent disadvantage in known combinations of colored transparent toner powders is accordingly that one or more mixed colors, e.g. one of the primary colors of red, green and blue, produced by subtractive mixing of the toner powders on the image receiving support, do not have the actually required color tint.
In addition to transparent colored toner powders, non-transparent colored toner powders are also known, e.g. colored magnetically attractable toner powders, as described in European patent application 075 346. These toner powders can also be used as single-component developer powders, i.e. without being mixed with carrier particles, in electro(photo)graphic printing processes and are also suitable for use in magnetographic printing processes in which latent magnetic images are developed. Multi-color printing can also be carried out in principle with non-transparent colored toner powders, but in order to achieve a complete color palette it would be necessary to use seven color separation images in red, green, blue, magenta, cyan, yellow and black, respectively. It is, however, a complex and expensive operation to construct a printer in which seven different color separation images are generated for transfer in register to an image receiving support.